1. Field of the Invention
The invention relates to a method and apparatus for storing a stream of video data on a storage medium. More particularly, the invention relates to a method of efficiently accessing video data stored on the storage medium.
2. Description of the Related Art
Video systems transmit, process and store large quantities of video data. To create a video presentation, such as a video movie, a rendering video system displays the video data as a plurality of digital images, also referred to as “frames,” thereby simulating movement. In order to achieve a video presentation with an acceptable video quality, or to enable transmission and storage at all, a conventional video system modifies the video data prior to transmission or storage. For instance, the video system compresses and encodes the video data to reduce the bit rate for storage and transmission.
In a conventional video system, a video encoder is used to compress and encode the video data, and a video decoder is used to decompress and to decode the video data. The video encoder outputs a stream of video data that has a reduced bit rate and a reduced redundancy. That is, the technique of video compression removes spatial redundancy within a video frame or temporal redundancy between consecutive video frames. The video encoder and video decoder usually operate in accordance with international standards, e.g., a compression process defined by the Moving Picture Expert Group (MPEG), e.g., MPEG-2, or by the International Telecommunications Union (ITU), e.g., the H.263 standard, that define uniform requirements for video coding and decoding. In addition, manufacturers of video coders and decoders modify or build upon the international standards and implement proprietary techniques for video compression.
The video system may store the compressed stream of video data on a storage medium, for example, a hard disk drive (HDD). The HDD may be included in a digital video recorder (DVR) that is coupled to a monitor or a television set in a viewer's home to record and to display program broadcast signals received via a coaxial cable, a satellite dish or an antenna for terrestrial radio frequency (RF) signals. The HDD may have a storage capacity of, for example, between 10 GB and 30 GB to selectively store, and thus record, up to 80 hours of compressed video and audio data. The recorded video data is then available for later reproduction when the viewer decides to watch the recorded video data.
The HDD includes several platters, wherein each platter records information in concentric circles called tracks. Each track is further broken down into smaller units called sectors, each of which may hold 512 bytes of user data. Each sector, however, actually holds more than 512 bytes of user data. Additional bytes are needed for control structures and other information necessary to manage the HDD, locate data and perform other “support functions”. For example, the bytes of a sector's synchronization field assist a drive controller to guide the read process. The sectors store the compressed stream of video data that is also broken down into smaller units, which the MPEG-2 standard defines as transport packets. The DVR reads from the HDD in continuous groups of sectors called clusters, wherein the HDD is addressable on sector boundaries.
Despite the existence of the international standards and the availability of sophisticated HDD technology, there is still a need for an improved technique for accessing video data stored on a HDD. In particular, when the DVR accesses the video data to read from the HDD, periods of stream discontinuity may exist during trick play functions such as Fast Forward, Reverse, Rewind and Skip. During these periods, the DVR may lose synchronization.